The present invention relates to a light output control circuit applicable for optical communication etc. having a driving circuit of a light emission element such as a semi-conductor laser (LD: laser diode) or a light emission diode (LED) and more particularly a light output control circuit for controlling constant light output level provided with a warning function of deteriorated light output.
In equipment such as optical transmission equipment which uses a light emission element, it is generally required to control light output to maintain at a predetermined value.
The efficiency of a light emission element such as an LD largely depends on temperature and also has a property of aged deterioration. It is therefore necessary to control to feed appropriate driving current to the light emission element in order to maintain light output level constant in any operating condition.
The APC (automatic power control) using negative feedback control has been introduced in a light output control circuit to control constant light output emitted by a light emission element.
In a light output control circuit to drive an emission element, a function is usually provided to monitor output power. A warning is issued when the output power decreases below a predetermined level. This facilitates to know appropriate time to replace the emission element caused by the breakdown or aged deterioration.
In FIG. 31, there is shown the first configuration example of a conventional light output control circuit to drive a light emission element. In this FIG. 31, pulse transmission data (DATA) is inputted with transmission input clock (CLK) to a D-type flip-flop (hereafter referred to as xe2x80x98DFFxe2x80x99) circuit 100, to be outputted to an LD driving circuit 101 after the pulse width of the input data (DATA) is corrected.
LD driving circuit 101, after receiving an output signal of DFF circuit 100, feeds a driving current to a laser diode (LD) to produce the LD emit light corresponding to the input data (DATA).
By using a light output control signal outputted from a comparator 102, a negative feedback function is carried out to produce stronger light emission when an LD light output is small, or to produce weaker light emission when the LD light output is large.
A photodiode (PD) receives a portion of the light outputted from the laser diode (LD) and converts the received light to the corresponding current to output. A current/voltage converter 103 converts the received current signal from the PD into a voltage signal.
A reference voltage generator 104 generates a reference signal from an input data (DATA) to output to comparator 102. It may also be possible that reference voltage generator 104 produces the reference voltage from a constant voltage generator.
A peak detection circuit 105 receives an output signal from current/voltage converter 103 to detect a peak value of the signal to forward to comparator 102. A capacitor is usually used in peak detection circuit 105 to detect the peak value.
Comparator 102 produces a differential value between the values of the reference signal from reference voltage generator 104 and the peak signal from peak detection circuit 105, then to feed to LD driving circuit 101 as a light output control signal.
Accordingly, the circuit shown in FIG. 31 performs the negative feedback control: a light output control signal from comparator 102 acts to increase an LD light output when the light output is small, and to the contrary to decrease the LD light output when the light output is large. Thus the LD light output is controlled to maintain a constant value.
In the above-mentioned first configuration of a conventional light output control circuit, there is shown in FIG. 32 an example of a circuit which outputs a warning signal of a deteriorated light output. In FIG. 32, the loop to control driving current illustrated in FIG. 31 is not shown.
Also in FIG. 32, current/voltage converter 103 and peak detection circuit 105 shown in FIG. 31 are integrated into one as a monitoring portion 106. A level comparator 107 compares an output of monitoring portion 106 to a threshold value which corresponds to the warning generation level led from constant voltage generator 108. When an output of monitoring portion 106 decreases below the threshold value from constant voltage generator 108, level comparator 107 outputs a warning signal of a deteriorated light output.
FIG. 33 shows the second configuration example of a conventional light output control circuit. Compared to the first configuration shown in FIG. 31, digital control is introduced in the driving current control circuit in FIG. 33 so that the circuit can easily be fabricated into an LSI.
In the configuration shown in FIG. 33, LD driving circuit 101 modulates driving current according to transmission data supplied by DFF circuit 100 to feed to the light emission element (LD). A peak value of the driving current (hereafter simply referred to as xe2x80x98driving current valuexe2x80x99) is controlled so that a driving current value is proportional to a digital value inputted to a digital-to-analog converter 110.
The above digital value is fed from a pre-stage counter 109, and therefore the produced driving current value is proportional to a value of counter 109. A photo diode (PD) for monitoring produces a monitoring current proportional to the light amount emitted from an emission element (LD). Then, the monitoring current value is converted to a voltage value in monitoring portion 106, to be compared in comparator 102 to a reference voltage 104 (i.e. a target value).
A counter value of counter 109 is changed using the result of comparison performed by comparator 102 in which a differential amplifier is used. Namely, when an output of monitoring portion 106 is smaller than the reference value 104, the counter value in counter 109 is increased by 1 to increase a driving current value. Also, when an output of monitoring portion 106 is greater than the reference value, the value in counter 109 is decreased by 1 to reduce the driving current. Through the operation of a negative feedback amplification described above, a light output is controlled to fix to the constant reference value.
Here, in the light output control circuit shown in FIG. 33, the driving current is controlled at the precision determined by the resolution which is fixed by the least significant bit (LSB) in digital-to-analog converter 110. For example, when digital-to-analog converter 110 is composed of 10 bits, the obtained resolution becomes 210=1024. When an output current to be controlled by this circuit ranges from 10 mA to 100 mA, a current source in LD driving circuit 101 is designed so that the LSB corresponds to 0.1 mA.
Also, the variation of digital values ranging from 100 to 1000 is so designed as to correspond to the variation of a driving current from 10 mA to 100 mA. Then, since the current 0.1 mA of the LSB corresponds 1% of the least value (10 mA) of the driving current, an accurate control having approximately 1% accuracy of driving current (also proportional to light output) can be achieved.
In the aforementioned second configuration of the conventional light output control circuit, a circuit to issue a light output deterioration warning may be similar to the deterioration warning circuit shown in FIG. 32 in the first configuration of the conventional light output control circuit shown in FIG. 31.
In an access system for optical communication system which has been in practical use in recent years, a burst transmission system is known as a required transmission system between subscribers and a switching office, in which data partitioned into cells are intermittently transmitted.
FIG. 34 illustrates an operational problem which may occur when the conventional light output control circuit shown in FIG. 31 or FIG. 33 is applied for the burst transmission system.
In FIG. 34, the horizontal axis and the vertical axis respectively denote the time and the driving current value. Here, the case of continuous light output is considered, as shown in FIG. 34A. Time is necessary after light emission element is driven to produce desired stable light output. In other words, an initial state (I) is required before a normal state (II) is attained.
In order to ensure the initial state in a light output control circuit of a burst transmission system, a starting cell SC may be inserted prior to a normal cell NC, as shown in FIG. 34B. However, a quite short time i.e. a few micro seconds corresponding to the duration of the starting cell SC is allowed before the light output reaches a target value.
Therefore, a longer rise time in addition to the duration of the starting cell SC is required. This may penetrate into the duration of the normal cell NC for communication, during which the light output can not be used for transmission.
To sum up, the following technical requirements must be satisfied for a light output control circuit to drive a light emission element to be used for a burst transmission system:
(1) To obtain quick response to reach the required light output.
(2) To control light output with high precision.
(3) To function APC (automatic power control) to cope with burst signals i.e. to maintain light output for a sufficient time during no-signal period between each burst signal, as well as to cope with the variation in the ambient temperature.
(4) To reduce the number of components externally attached to a semiconductor integrated circuit.
In the first configuration example of a conventional light output control circuit shown in FIG. 31, the above requirement (1) may be realized if the response speed to detect peak value improves in peak detection circuit 105. This may be achieved by providing a capacitor of smaller capacitance in peak detection circuit 105 and by feeding a larger charging current. Also, the above requirement (2) may be realized by increasing negative feedback gain.
A problem exists, however, that in order to realize the above requirement (3), light output must be maintained for a long time by providing larger capacitance to maintain peak value in peak detection circuit 105. This may practically damage a quick response characteristic required in the above (1).
Furthermore, a capacitance leak in a capacitor to maintain peak value and a leak current in an FET used in the circuit may damage the high precision control for the above requirement (2). Moreover, it is difficult to compensate light output when abrupt change of the ambient temperature occurs between burst signals.
To satisfy the above requirement (4), it is required to maintain light output for a long time, for example for a few milliseconds, by providing a capacitor having capacitance of several micro farads. This necessitates attaching an outer component because of difficulty in fabricating such large capacitor in an LSI, and makes it difficult to compose smaller circuit configuration.
As having discussed above, it is difficult to satisfy all the above four requirements (1) to (4) at a time when using the first circuit configuration of the conventional light output control circuit shown in FIG. 31.
As a method to realize the above four requirements (1) to (4), the second circuit configuration of the conventional light output control circuit shown in FIG. 33 may be chosen. Hereafter, an operational characteristics shown in FIG. 35 is discussed with regard to the second circuit configuration of the conventional light output control circuit shown in FIG. 33. In FIG. 35, the horizontal and the vertical axes denote the time and the driving current value, respectively.
At the start of operation, the value of counter 109 has already been reset. When the aforementioned feedback operation is started, the value of counter 109 is updated by either adding or subtracting 1 at each step. This is carried out according to the result of comparing an output of monitoring portion 106 with the reference value 104. Accordingly, the driving current value is updated to make the driving current change stepwise.
In this case, the driving current value changes by the step of 0.1 mA corresponding to the least significant digit (LSB) of a digital-to-analog converter. When the driving current reaches the target value, the driving current value drifts between the two values centered by the target value. Thus a stable light output is obtained corresponding to the driving current within this range.
According to this conventional technique, a counter value is increased by 1 (at the unit of the LSB) from zero at the start until the desired light output is produced. Therefore, in the case a larger current (i.e. larger digital counter value) is required to produce a desired light output, a larger number of steps are necessary, resulting in a longer rise time.
For example, in order to obtain the maximum driving current of 100 mA in the case a 10-bit digital-to-analog converter 110 is used, the counter value started from zero has to be updated 1024 times in the worst case to produce desired stable light output.
Also in the case of continuous transmission, data cannot be transmitted during the startup period shown as the initial state (I) in FIG. 34A. To cope with this problem, it is required either to lengthen the startup time of an overall system, or to shorten the timing margin of the system.
In the conventional light output control circuit, there is a defect that a light output value cannot be fed back in the burst transmission method where no-data period continues for a long time. To cope with this, in the conventional digital light output control circuit of data-synchronous type, the light output update timing is determined by checking the existence of input data.
The deteriorated light warning circuit shown in FIG. 32 has also been used in the second configuration of a light output control circuit shown in FIG. 33.
As a concrete example of the second light output control circuit shown in FIG. 33, there has been a method to control to update a driving current (i.e. to update light output) synchronously when successive n bits of xe2x80x981xe2x80x99 appear in data for transmission.
In such a configuration, there may be cases that the monitoring signal level for data having less than successive n bits of xe2x80x981xe2x80x99 is deteriorated, compared to the intrinsic value, caused by the insufficient bandwidth of current/voltage converter to convert a monitoring current to a voltage signal.
FIG. 36 illustrates such undesirable condition as described above. In FIG. 36, an example of data is shown for one burst period.
In the case a warning circuit of a deteriorated light output shown in FIG. 32 is applied, a false warning may possibly be issued in the period (I) and period (II), as shown in FIG. 36, when data having less than successive n bits of xe2x80x981xe2x80x99 (3 bits in the example shown in FIG. 36) is inputted.
Namely, in the period of (I) and (II) in FIG. 36, the respective number of successive xe2x80x981xe2x80x99 is one bit. Therefore, as mentioned above, a peak value (B) of a monitoring voltage output (A) cannot exceed a threshold voltage (C) because of an insufficient bandwidth of monitoring portion 106.
This results in issuing a false warning of light deterioration because level comparator 107 determines the light output is below the reference value 108. On the other hand, in the period (III) having more than three successive bits of xe2x80x981xe2x80x99, a warning is not issued because the peak value (B) of the monitoring voltage output (A) exceeds the threshold voltage (C). In the latter zone of period (III), the number of successive bits of xe2x80x981xe2x80x99 is one, but the peak value (B) gradually decreases and a warning is not issued until the peak value (B) falls below the threshold value (C).
FIG. 37 illustrates that a problem arises between burst signals in the case of burst signal transmission when a conventional warning circuit of a deteriorated light output shown in FIG. 32 is applied.
In FIG. 37, the N-th burst and the (N+1)-th burst are shown as transmission data (A). Light output (C) is produced only during a burst period using a transmission/reception switch signal (B). Accordingly, a monitoring current (D) is outputted while the light output (C) appears. However, if a time constant to maintain a peak value in monitoring portion 106 is insufficiently small, the peak value of a monitoring voltage from monitoring portion 106 gradually decreases after the burst signal is suspended, and falls below the threshold value before the succeeding burst signal appears.
Therefore, in this case, a false warning is issued between the period of burst signals (as shown in FIG. 37G). In order to avoid such malfunction of false warning which arises in the intervals of burst signals, a measure is taken so that a deterioration warning output is masked during non-existent period of burst signals using a transmission/reception switch signal. Thus a false warning may be prevented (refer to FIG. 32 and FIG. 37F).
However, there may be a case depending on a system that a relevant process is not carried out outside the light output control circuit of the equipment, as shown in FIG. 37. Also there may be a case that an external signal is not inputted to the LSI. In such cases, the conventional warning circuit of a deteriorated light output cannot be used, and measures against the following issues are required:
(1) To cope with a light output control circuit having light output updated synchronously with input data, and
(2) To cope with a burst transmission system even in case no appropriate measure is taken outside the circuit of the equipment, or no external signal applied to switch transmission/reception condition. (i.e. to prevent from issuing any false warning of a deteriorated light output in the intervals of burst signals.)
It is an object of the present invention to provide a light output control circuit which may be applied to a burst transmission system, having an unerring warning output function of deteriorated light in a light output control circuit in which light output is updated synchronously with input data.
As the basic configuration of a light output control circuit of the present invention to attain the above object includes; a monitoring portion to detect light output from a light emission element which is driven in accordance with data to be transmitted; a level comparator to compare a monitoring signal from the monitoring portion with a reference signal; a data detection portion to detect the existence of data to be transmitted; and an output control portion to determine to output a warning using an output signal of the data detection portion and the level comparator.
As the second configuration of the invention, the data detection portion in the above basic configuration outputs a detected signal of xe2x80x98existence of transmission dataxe2x80x99 when successive n bits of xe2x80x981xe2x80x99 is detected in the transmission data.
As the third configuration of the invention, the data detection portion in the above basic configuration outputs a detected signal of xe2x80x98no existence of transmission dataxe2x80x99 when successive n bits of xe2x80x980xe2x80x99 is detected in the transmission data.
As the fourth configuration, the output control portion in the above basic configuration includes; a delay circuit to delay an output signal of the data detection circuit; and a flip-flop to input into a data terminal an output of the level comparator using an output of the delay circuit as a clock signal.
As the fifth configuration, the output control portion in the above second configuration includes; the first AND gate to produce logical product of an output of the data detection portion and an output of the level comparator; an inverter to invert an output signal of the level comparator; the second AND gate to produce logical product of an output signal of the data detection portion and an output signal of the inverter; a latch circuit to which an output of the first AND gate is inputted as a set signal and an output of the second AND gate is inputted as a reset signal.
As the sixth configuration, the output control portion in the above third configuration includes; the first NOR gate to produce inverted logical sum of an output signal of the data detection portion and an output of the level comparator; an inverter to invert an output of the level comparator; the second NOR gate to produce inverted logical sum of an output signal of the data detection portion and an output signal of the inverter; and a latch circuit to which an output of the second NOR gate is inputted as a set signal and an output of the first NOR gate is inputted as a reset signal.
As the seventh configuration, in the above second configuration or the above third configuration, a delay circuit is provided between the data detection portion and the output control portion in order to synchronize phases between the transmission data and an output of the level comparator. Accordingly, the input timing of an output of the data detection portion to the output control portion coincides with the input timing of an output signal of the level comparator to the output control portion.
As the eighth configuration, the delay circuit in the seventh configuration includes a timer in which the counting is originated by an output signal of the data detection portion.
As the ninth configuration, in the second configuration or the third configuration, the threshold level of the reference signal to be inputted to the level comparator varies according to an output state of the output control portion provided with a hysteresis characteristic.
As the tenth configuration, in the second configuration or the third configuration, a reset signal generator is provided by which a reset signal forcibly resets the output state of the output control portion.
As the eleventh configuration, the reset signal in the tenth configuration is generated when either transmission data or clock signals do not exist for a certain period.
As the twelfth configuration, when the light output control circuit is constituted by an LSI, the reset signal in the tenth configuration is a signal supplied from outside the LSI.